A FM-CW radar can measure the distance between vehicles and the relative velocity with, e.g., millimeter waves of 76 GHz without serious interference from the weather or the time of day with high precision. Therefore, the FM-CW radar has started to become partly provided for vehicles. The measurement principle is well-known. Based on the measurement principle, triangular modulation waves of 76 GHz as mentioned above are transmitted, the distance can be measured from the time delay (return time of reflection waves from a target) of reception waves, and the relative velocity can be measured on the basis of the frequency shift (the amount of Doppler shift) between transmission waves and reception waves due to the Doppler effect.
Specifically, the frequency shift and a time delay (Δt) are caused in the reception waves after the transmission waves are transmitted. Herein, the time delay (Δt) denotes the distance to the target, and the frequency shift denotes the relative velocity from the target. Further, a distance R to the target is obtained as follows.2R=Δt×C (where C=light velocity), i.e., R=Δt×C/2  (A)
Herein, the relative velocity is 0 and reference symbol fr denotes a beat frequency. Thus, as the time delay Δt increases, the beat frequency fr is increased. Specifically, the following well-known relationship is satisfied.Δt=fr/(2×ΔF×fm)(where ΔF=modulation width of transmission waves and fm=repeating frequency)
This is substituted for (A) and the following relationship is obtained.R=C×fr/(4×ΔF×fm)The distance R can be obtained on the basis of the beat frequency fr.
In the above description, the amount of Doppler shift is 0 (that is, the relative velocity is 0). Further, when the relative velocity is not 0, the following expression can be applied.fr=(fb1+fb2)/2(where fb1=beat frequency at the rising interval of the triangular modulation waves and fb2=beat frequency at the falling interval of the triangular modulation waves).
When reference symbol fo denotes a carrier frequency of the transmission waves, the relative velocity V can be obtained as follows.fd=(fb1−fb2)/2In this case, the relative velocity V can be obtained from the following expression.V=C·fd/(2−fo)
As a consequence thereof, for the FM-CW radar, the triangular modulation waves with high linearity with a modulation width of 100 MHz need to be generated and further be modulated.
In order to obtain, for the FM-CW radar, the triangular modulation waves with high linearity, the following three methods can be considered.
1. The sensitivity for modulating the frequency uses a constant VCO.
2. As disclosed in Patent Document 1, linear modulation waves are obtained by distorting a modulation signal in the reverse direction so as to set the constant sensitivity for modulating the frequency.
3. As disclosed in Patent Document 2, an oscillation frequency is monitored in realtime and, when the frequency is shifted, the frequency is corrected by using a closed loop.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 8-18343 (publication date: Jan. 19, 1996)
Patent Document 2: Japanese Unexamined Patent Application Publication No. 6-120735 (publication date: Apr. 28, 1994)
The realization of method 1 needs VCO with high modulating characteristic. However, the VCO cannot be realized.
Further, the method 2 uses a low-frequency circuit, thereby relatively easily obtaining preferable linearity. However, the method is based on the open-loop correction. Thus, the change in VCO characteristic causes a problem of the shift of characteristic. A correcting circuit increases costs.
Furthermore, a typical one of the method 3 is PLL (Phase Locked Loop). The use of the PLL enables the relatively easy acquisition of the triangular modulation waves with high linearity and also enables the correction even with the aging change in VCO characteristic. However, the PLL has such a problem that a reference oscillator and a frequency divider are necessary and the structure is thus complicated with increased costs. In addition, millimeter waves need to be converted-down for the purpose of the comparison with the reference signal, and a circuit for the converting-down operation is complicated and the costs are further increased. A method of FLL (Frequency Locked Loop) is similar to the PLL. However, the FLL needs the reference oscillator and the costs are increased.
As a consequence thereof, all the above-mentioned methods have technology problems and increase the costs.